Air compressors are becoming commonplace in home workshops. In general, an air compressor is a machine that decreases the volume and increases the pressure of a quantity of air by mechanical means. Air thus compressed possesses great potential energy, because when the external pressure is removed, the air expands rapidly. The controlled expansive force of compressed air is used in many ways and provides the motive force for air motors and tools, including pneumatic hammers, air drills, sandblasting machines, paint sprayers, and others.
A conventional home workshop air compressor includes a storage tank for compressed air, and a prime mover mounted on the compressor tank for compressing the air flowing into the compressor tank. The prime mover may be a gas engine or an electric motor, but most conventional home workshop models utilize electric power.
The basic components of an electric air compressor are an electric motor, a pump, a pressure switch, and a tank. The electric motor powers the pump. The pump compresses the air and discharges it into the tank. For conventional air compressors, compressed air from the pump is discharged through a tube and a check valve into the tank. The check valve prevents air from flowing out of the tank back through the tube when the compressor pump is not in operation. The tank stores the compressed air.
The pressure switch shuts down the motor and relieves air pressure in the pump and transfer tube when the air pressure in the tank reaches an upper level limit, or cut-out pressure. As the compressed air in the tank is used and the pressure level in the tank drops to a lower level limit, or cut-in pressure, the pressure switch restarts the motor automatically and the pump resumes compressing air.
Conventional air compressors include a tank pressure gauge that measures the pressure level of the air stored in the tank. This gauge is not adjustable by the operator, and does not indicate line pressure. A separate line pressure gauge is provided for indicating the output pressure. An air pressure regulator is provided to allow a user to adjust line pressure to the tool that is being used. In conventional home style or workshop air compressors, the air pressure regulator utilizes a fixed rate spring and a variable knob. By screwing the knob inward, the force the fixed spring applies to the regulation valve increases. This increase of force opens the regulation valve and increases the output of pressure of the air compressor.
Although conventional air compressors work well for their intended purpose, the existence of both the tank pressure and line pressure gauges may be confusing to a new user. The variable knob and fixed rate spring may also be confusing, and may be difficult to adjust to a desired output pressure.
Another problem inherent in the design of the mechanical gauges is that the gauges are susceptible to vibration, which all air compressors have. The amplitude of the vibration varies with the design of the compressor. Vibration sometimes makes the mechanical pressure gauges on conventional air compressors difficult to read.
One downside to electrical air compressors is that they must be designed to operate at conventional circuit levels. Most electrical air compressors operate on standard household electrical circuits that in the United States are typically rated at 120 volts and 15 amps. Less common but still applicable are 120 volts, 20 amp, and 240 volt, 15 amp circuits. To prevent overload, air compressors are designed to operate at their maximum load point within the least common denominator of these circuits.
Designing a conventional air compressor within the limits of existing circuits can cause limitations in the performance of a conventional air compressor. Conventional air compressors have fixed speed motors. A typical operating characteristic of conventional air compressors, because they have fixed speed motors, is that the load on the motor varies as the machine runs through its operating pressure. While the pump operates at nearly the same speed throughout its range of operation, the load on the motor varies significantly. Higher pressures require more power to run the pump, and result in loading the motor to higher horsepower levels. The higher horsepower levels correspond to increased amperage. The air compressor must be designed so that it can operate at the increased amperage level without tripping a circuit. Since the air compressor is limited to an electric circuit of a certain size, the overall performance of the machine is limited based on the peak amperage used at the maximum load.
Due to manufacturing tolerances causing some degree of variation in the load from air compressor to air compressor, most conventional air compressors are not designed at the absolute maximum performance (i.e., 15 amps). As in conventional fixed speed air compressors, the nominal rating would be somewhat less so that all machines would fall within an acceptable range, such as 14.2 to 14.9 amps. Thus, many air compressors are not capable of drawing amps that are available for the air compressor.